1. Field of the Invention
The present invention relates to a one-way clutch that transmits the rotation of a drive member in only one direction to rotation of a driven member.
2. Description of Related Art
Ratchet-style clutches that transmit rotation in only one direction are known. FIGS. 7 and 8(a)-8(b) illustrate such a clutch. In FIGS. 7 and 8(a)-8(b), a drive clutch plate 102 and plate 105 are connected together through hole 102a and spindle 105a so that they can rotate relative to each other. A ratchet claw 103 is provided on a fixed spindle 102b of the drive clutch plate 102. A ratchet arm 103b of ratchet claw 103 contacts a fixed spindle 102c of the drive clutch plate 102. A ratchet claw tooth 103a of ratchet claw 103 engages the cam surface 105c of the secondary driven plate 105 to enable drive plate 102 to drive secondary driven plate 105 as shown in FIG. 8(a). The ratchet claw tooth 103a is pressed against a stepped portion 105(b) of cam surface 105c with a force determined by the resilient force created by the elastic deformation of the ratchet arm 103b. As shown in FIG. 8 (a), when the drive plate 102 rotates in the counter-clockwise direction (shown by arrow A) the stepped portion 105b of the cam surface 105c and the ratchet claw tooth 103a engage to transmit the rotation of the drive plate 102 to the secondary driven plate 105. As shown in FIG. 8 (b), when the drive plate 102 rotates in the clockwise direction (shown by arrow B) the ratchet claw 103 slides past the cam surface 105c, thus allowing the drive plate 102 to rotate without engaging the secondary driven plate 105.
With the clutch described above, friction is generated between the ratchet claw tooth 103a and the cam surface 105c because the ratchet claw tooth 103a of the ratchet claw 103 is pressed against the cam surface 105c by the resilient force of the ratchet arm 103b. Thus, even when the drive plate 102 is rotating freely as shown in FIG. 8(b), some torque is transmitted to the secondary driven plate 105. Simultaneously, a small torque load also appears in the drive plate 102. Hereafter, the torque load of the drive plate 102 during free rotation will be referred to as the free rotation load torque, and the torque transmitted to the secondary driven plate 105 will be referred to as the free rotation torque.
Especially when the clutch described above is used with small precision equipment such as cameras, comparatively large free rotation torque and free rotation load torque are generated when the ratchet claw tooth 103a rides over the stepped portion 105b because the bending length (length in the perimeter direction) of the arm 103b cannot be made sufficiently long in relation to the height of the stepped portion 105b. The pressure of the ratchet claw tooth 103a against the cam surface 105c is designated to be slightly larger than that necessary for proper operation to assure that the claw 103a will positively press against the cam surface 105c, regardless of variations in manufacture of the clutch. Manufacturing differences related to a larger design dimension will cause the pressure to be correspondingly larger than the design pressure.